1. Field of the Invention
The present invention relates to a switching element which operates in response to a change in applied voltage.
2. Description of Related Art
Electric conductivity in some materials changes drastically when an applied voltage changes. One of such materials is Cu-TCNQ, a copper complex of 7,7,8,8-tetracyanoquinonedimethane or TCNQ. Cu-TCNQ is a charge transfer complex including copper which serves as an electron donor and TCNQ which serves as an electron acceptor. When a voltage applied to the Cu-TCNQ is swept from a low-voltage side to a high-voltage side, the value of current increases drastically at a certain threshold voltage. Such a switching characteristic of the Cu-TCQN is disclosed in the article xe2x80x9cElectrical switching and memory phenomena in Cu-TCNQ thin filmsxe2x80x9d (R. S. Potember et al., Applied Physics Letter 34(6), pp. 405-407, Mar. 15, 1979).
The switching characteristic of Cu-TCNQ is expected to be utilized in a wide range of electronic devices. In recent years, with ongoing miniaturization of electronic devices, there is a concomitant trend of increasing density in components. In order for Cu-TCNQ to be used in such miniaturized electronic devices, film thickness of Cu-TCNQ must be decreased.
Conventionally, Cu-TCNQ for example is made in the form of a thin film by means of solution spreading method as disclosed in the article. In the solution spreading method, acetone and acetonitrile is mixed at a volume ratio of 1:1 for example, to prepare a solvent. This solvent is saturated with TCNQ, and a Cu substrate is soaked into the saturated solution. Then, charge transfer occurs from Cu to TCNQ on surfaces of the Cu substrate, resulting in formation of Cu-TCNQ. As the complex grows on the Cu substrate, a film of Cu-TCNQ is formed on the Cu substrate. This Cu-TCNQ is a polycrystalline film including a plurality of crystalline. Then, a thin film of Al is formed on the obtained Cu-TCNQ film. The result is a multilayer element including a Cu substrate, a Cu-TCNQ film and an Al film.
The above multilayer element is known to have a switching characteristic to be described below. Specifically, the Al film is used as an upper electrode, the Cu film is used as a lower electrode, and a voltage applied between the electrodes is swept. In this operation, at a certain threshold voltage, electric resistance of the Cu-TCNQ film which is in a high state changes to a low state. On the other hand, when the Cu-TCNQ film which is in the low resistance state is given a reverse bias voltage greater than a certain threshold value, the Cu-TCNQ film returns to the high-resistance state. As described in the article mentioned earlier, the following consideration has been made for this switching phenomenon: The high-resistance state appears when the Cu takes the state of cation radical and the TCNQ takes the state of anion radical, whereas the low-resistance state appears when the Cu takes the state of neutral atom and the TCNQ takes the state of neutral molecule. However, the switching mechanism is not yet known.
The Cu-TCNQ film made in the solution spreading method is a polycrystalline film formed of relatively large crystals, and therefore is low in film uniformity. This non-uniformity in the film causes various problems. For example, when this Cu-TCNQ film is incorporated in an element, the switching action is not reproduced. Specifically, a Cu-TCNQ film formed by means of the solution spreading method can take, as has been described above, a high-resistance state and a low-resistance state as an applied voltage changes, and then come back to the high-resistance state. However, when the voltage applied to the Cu-TCNQ film is swept thereafter, the switching does not take place. (The switching function is lost.) Another problem is that the Cu-TCNQ film is poor in stability. Because of these problems, no market has ever seen electronic devices incorporating a switching element provided by a conventional Cu-TCNQ film formed by means of the solution spreading method.
Another problem is that according to the solution spreading method it is difficult to control the thickness of the growing film. For example, when a Cu substrate is soaked into a TCNQ-saturated solvent to form a Cu-TCNQ film, control on the thickness of the Cu-TCNQ film is performed by taking the soaked Cu substrate out of the saturated solution in a certain predetermined amount of time. However, in such a method of control on the film thickness, it is difficult to obtain a desired thickness of the film. In addition, in the solution spreading method, the Cu-TCNQ film is formed as a polycrystalline film including relatively large crystals. For this reason, the control of the film thickness is possible only in the order of micron meters.
The present invention was made under the above circumstances, and it is therefore an object of the invention to eliminate or reduce the conventional problems. A switching element provided by the present invention has a reproducible switching characteristic, and can be as thin as appropriately applicable to electronic devices. Further, a method provided by the present invention is suitable for a manufacture of such a switching element.
A first aspect of the present invention provides a switching element. This switching element includes a first electrode layer, a second electrode layer, a switching layer and an insulating layer. The switching layer includes a charge transfer complex containing an electron donor and an electron acceptor and is provided between the first electrode layer and the second electrode layer. The insulating layer contacts the switching layer between the first electrode layer and the switching layer. The switching layer switches from a high-resistance state to a low-resistance state upon application of a voltage greater than a first threshold value in a first bias direction between the first electrode layer and the second electrode layer, maintaining the low-resistance state when the applied voltage decreases thereafter beyond the first threshold value, and likewise, switches from the low-resistance state to the high-resistance state upon application of a voltage greater than a second threshold value in a second bias direction or a reverse direction to the first bias direction, maintaining the high-resistance state when the applied voltage decreases thereafter beyond the second threshold value.
Preferably, the switching element further includes an additional insulating layer between the switching layer and the second electrode layer.
Preferably, the first bias direction is a direction of voltage drop from one of the first and the second electrode layers to the other.
Preferably, the electron acceptor is provided by an organic compound having a pi electron system.
Preferably, the electron acceptor is provided by TCNQ or a derivative of TCNQ.
Preferably, the electron donor is provided by a metal. The metal is selected from a group consisting of Cu, Ag and K.
Preferably, a presence ratio of the electron donor to the electron acceptor in the switching layer is not smaller than a half and not greater than three seconds.
Preferably, the switching layer contains an amorphous structure.
Preferably, the insulating layer contains an oxide. The oxide is provided by Al2O3 or SiO2.
Preferably, the second electrode layer contacts the switching layer, and contains at least one of Al, Mg and Ag.
A second aspect of the present invention provides a method of making the switching element according to the first aspect. The method includes: a first-electrode layer forming step for formation of a first electrode layer on a substrate; an insulating layer forming step for formation of an insulating layer on the first electrode layer; a switching layer forming step for formation of a switching layer by depositing an electron donor material and an electron acceptor material on the insulating layer; and a second-electrode layer forming step for formation of a second electrode layer.
The method further includes a step of forming an additional insulating layer on the switching layer after the switching layer forming step.
Preferably, the first-electrode forming step uses a vacuum deposition method or a spattering method for deposition of a first electrode material on the substrate, and the deposited electrode material in the first-electrode forming step is exposed to the atmosphere in the insulating layer forming step.
Preferably, a surface of the first electrode layer formed in the first-electrode forming step is subjected to a UV ozone treatment in the insulating layer forming step.
Preferably, the switching layer forming step uses a vacuum deposition method for deposition of the electron donor material and the electron acceptor material on the insulating layer.
Preferably, a ratio of a deposition rate of the electron donor material to a deposition rate of the electron acceptor material in the switching layer forming step is not smaller than 1.5/0.7 and not greater than 1.9/0.3.
Preferably, the method further includes a curing step of leaving the element under vacuum after the second-electrode layer forming step.
Preferably, the method further includes an aging step for application of a voltage between the first electrode layer and the second electrode layer.
Other characteristics and advantages of the present invention will become clearer from the description of preferred embodiments to be made here below.